In the related art, fan-angle nozzles (FWD) or fan nozzles are used parallel or perpendicular to the wall that is plastered or surfaced with mortar on the inside of heat exchangers for cement cylindrical rotary kilns. The cakes of cement raw meal that regularly build up are removed by air blasts so that an optimum heat exchange process/material flow is achieved. In addition, the alternative of labor-intensive, high-risk manual removal involving the insertion of air lances or pokers through openings in the heat exchanger wall is avoided. If the mouthpiece of the fan nozzle becomes worn or eroded by chemical reactions, the large-surface cleaning effect decreases. Then, or during regular kiln maintenance, at the latest, the person setting up the heat exchanger must remove the block material and the old nozzle, weld in a new nozzle and fill the hollow chamber with fireproof cement.